Michelle and Lavinia Osbourne discovered they were half-siblings owing to a rare event when two eggs released in the same cycle are fertilised by different men

The finding means the sisters are not biological twins but half-siblings, the result of a phenomenon known as heteropaternal superfecundation, in which two eggs released during the same ovulation cycle are fertilised by sperm from different men.

Although documented only about 20 times worldwide, specialists believe such cases may go undetected unless there is a reason to question paternity.

Allan Pacey, professor of andrology at the University of Manchester, said he had never encountered a similar case in more than three decades working in reproductive health.